Since the introduction of automatic transmissions, hydrodynamic torque converters have been the connecting link between a prime mover and the transmission proper. Because of the slip, a converter makes a comfortably smooth start possible. At the same, it time absorbs irregularities in the rotation of the combustion engine. Additionally, a great starting torque is made available by the torque increase conditioned by principle.
Strictly speaking, a converter generally comprises a converter housing, an impeller, a stator and a turbine wheel. Because of the torque being transmitted by the hydrodynamic forces from the impeller, via the stator, to the turbine wheel, slip results between the impeller and the turbine wheel. This causes a loss in efficiency.
To improve the efficiency, a bridge clutch was introduced which bridges the converter at certain rated speeds. The bridge clutch can be fitted before the turbine wheel, that is, between the converter housing and the turbine wheel, or behind the impeller, that is, between the impeller and another transmission unit.
The bridge clutch's disadvantage is the loss of the absorption of the vibration generated by the differential speed between the turbine wheel and impeller. To compensate for this, an additional shock absorber must be applied or the clutch itself must be designed as a shock absorber.
As a consequence of limited space conditions within the converter and of the complicated vibration systems of an automatic transmission, mechanical torsional shock absorbers cannot be designed so that the bridge clutch can be engaged already at low engine speeds and in the lower gears. Otherwise low, dull booming sounds will emanate throughout the body. In modern torque-optimized engines with high engine torques, at speeds slightly above the idling speed rigid power trains must already be used to prevent twisting thereof. Thereby the inherent frequency is moved to higher speeds, thus intensifying the problem of booming noises in the body.
Powerful transverse engines, having great capacity, increasingly require a narrow converter design. A mechanical torsion absorber thus creates considerable problems.
A bridge clutch, with regulated slippage, already makes engaging the converter at low driving speeds possible, absorbing the vibrations in critical speed ranges by the bridge clutch slip, reducing the cost of the torsional shock absorber and further reducing the consumption of benzene.
Torsional vibrations are generated in the drive train as a result of an angular acceleration of the crankshaft followed by a delay due to the compression in the next cylinder during each ignition of the combustible mixture in a cylinder. The angular speed thereby fluctuates between a maximum and a minimum.
As the engine speed increases, the torsion irregularity decreases proportionally 1/n. At a speed of typically about 2,000 1/min, values of speed fluctuation are reached which scarcely decrease further as the engine speed increases. The curve of the angular deflection is proportionally 1/n while the vibration range of the angular acceleration is almost independent of speed. Because of this, as a rule, above about 2,000 engine revolutions the bridge clutch can remain engaged during traction operation. Below 2,000 engine revolutions, the torsion irregularity suddenly increases so that the bridge clutch must be operated disengaged or adjustedly slipping.
During the engine coasting operation, the gas pressure in the cylinder is substantially less than during a traction operation, whereby the critical range moves to higher speeds. The vibration range of the angular acceleration thereby intensively increases as the engine speed increases. Body booms in the overrun are therefore detected mostly at speeds way above 2,000 1/min. For a comfortable drive the bridge clutch must, in this case, be disengaged or regulated.
In order to achieve a sufficient absorption of the torsion vibration, a slip of up to about from 2% to 3% is needed. A slip or more than from 2% to 3% hardly produces any further increase in absorption.
If the interior of the converter is divided in two spaces by a piston of the hydraulic clutch (a smaller space between the hydraulic clutch piston and the converter housing and a larger space between the piston and the impeller being thus formed) the regulation can be obtained by passing an increased pressure into the turbine space of the pump and adjustedly controlling the pressure in the clutch piston space of the converter lid.
In addition, regulation can be achieved by venting the small converter space and adjustedly controlling the pressure of the pump and the turbine space.
A switching logic for a bridge clutch can look, for example, as if a control pressure acts upon valves which then vent a line to the smaller converter space and pass the main pressure into the larger converter space whereby the piston of the bridge clutch is engaged.
Control of the bridge clutch is obtained by a pressure control function resulting from a force balance of control pressure, spring tension and operative regulating pressure of a differential piston. To adjust a constant differential speed on the bridge clutch, the clutch pressure is adjusted proportionally to the torque by the control pressure.
A function of the regulated bridge clutch is to absorb, by as small a slip as possible, the torsional vibrations in the input so that no booming or humming can be heard.
This can be achieved, for example, by determining the slip required by characteristic lines which are determined by tests of the specific vehicle engine, or the slip depending on rotational irregularities, or the slip acts based on a constant nominal value which is fed to a control circuit.
The stated basic problem consists in controlling the high frictions appearing in bridge clutches.
Additionally, the problem is made worse by new driving strategies in which the bridge clutch, already in the range of high conversions, is engaged in order to save fuel and improve driving properties such as the direct response to a change of the accelerator pedal position.
The problem of higher friction work is in the considerations regarding substituting a hydraulic starting clutch for a converter.
The friction value cover is determined by the oil and the friction lining.
If, in one area of the bridge clutch, a temperature peak is reached which leads to oil damage, the friction value curve is thereby changed and as the slip increases the friction value m decreases in comparison to the normal rising curve.
As a result, the increase of the slip is no longer compensated by a simultaneous increase of the friction value, so no stable operating point appears. Also, friction vibrations can generate due to damaged oil.
Therefore, the problem basically consists in locally preventing high temperature peaks and altogether keeping the temperature below the critical temperature at which the oil will be damaged.
It is known to provide friction linings with grooves of different kinds and to cool them with the largest possible oil current passed through the grooves. With the guided structure of the shape of the grooves, an even high heat dissipation is obtained (see VDI-Report No. 649, 1987, pp. 335-358).
In cases of high heat development, the heat is not sufficiently dissipated in all zones of the friction lining, since the friction lining is not sufficiently flowed through. The temperature peaks caused thereby damage the oil.
Besides, the flow rate of the oil depends on the wear of the friction linings since the through flow cross section of the grooves decreases with the wear of the friction lining.
Moreover, the flow rate of the oil depends on the actuating pressure of the bridge clutch whereby, at elevated actuating pressures, the pressure effective for the contact force drops and the transmissible forces of the bridge clutch becomes weaker.
Therefore, the problem to be solved by the invention is to enable a high and even heat dissipation over the whole friction surface and permit an oil flow rate independent of the wear of the bridge clutch and largely of the actuating pressure.